John Legend is an artist mature beyond his years. He is a crooner in the best possible sense of the word — self-assured, gentle and debonair. For Legend’s sophomore album Kanye West resumes his role as executive producer, but Once Again is a mellower affair than Get Lifted. The songs get their gentility from Legend’s low, cocoa-smooth vocals, which never fail to bring a potentially hyper song to a place of calm. Though Once Again features an impressive assembly of producers — including Craig Street, will.i.am and West (all contributed multiple songs) — Legend is wholly in control of the album’s tone and tempo. The hip-hop underpinnings are balanced by the elegant jazz of “Again” and “Maxine.” “Where Did My Baby Go” is so simple that it feels timeless — it could be a piece of an old album by Arthur Alexander or even Joan Baez, made new again by Legend’s warm breath. “Save Room” is a song that glides so sweetly and effortlessly that it challenges every shred of malice that might be lurking within the listener. Even sweeter is “Slow Dance,” a melody as compact and pretty as a jeweled pocketwatch.